Method of making sintered metal-diamond aggregates

ABSTRACT

A method of making aggregated diamond abrasive particles comprising forming a mixture of diamond abrasive particles and a powdered metal, heating the mixture to sinter the metal followed by cooling to produce a mass and crushing the mass to obtain the aggregated particles, characterized in that the particles are selected from Types A, B and C, as hereinafter defined, and mixtures thereof: 
     Particles of Type A have the following characteristics: 
     1. A Friatest Index of about 70 to 90 for those particles of size 74 to 88 microns. 
     2. Medium metal content. 
     3. Predominantly translucent color with the occasional transparent white, grey and yellow particle. 
     4. Predominantly blocky shape tending towards elongate particles. 
     5. Predominantly rough and undulating surface. 
     Particles of Type B have the following characteristics: 
     1. Weak, having a Friatest Index of about 65 to 88 for those particles of size 74 to 88 microns. 
     2. A high metal content. 
     3. Predominantly dark in color. 
     4. A blocky and irregular shape. 
     5. Rough with surface irregularities, re-entrant angles and etch pits. 
     Particles of Type C have the following characteristics: 
     1. Medium strength, having a Friatest Index in the range about 100 to 121, for those particles of size 74 to 88 microns. 
     2. Low metal content. 
     3. Predominantly yellow in color. 
     4. An angular to blocky shape with sharp edges. 
     5. A predominantly smooth surface. 
     The particles so produced are particularly useful in the grinding, particularly dry grinding, of carbides, nitrides and abrasive compacts.

This invention relates to abrasive materials and more particularly to aggregated diamond abrasive particles.

U.S. Pat. No. 4,024,675 describes a method of producing aggregated abrasive grains by sintering a mixture of abrasive powders, metal powders such as copper, tin, nickel, cobalt, iron, silver and alloys thereof and an adhesion-active agent such as titanium, zinconium, chromium, vanadium and silicon to produce a porous cake and then crushing the sintered cake to produce the aggregated particles. The abrasive particles used in the method are selected from diamond, cubic boron nitride, boron carbide, silicon carbide, aluminium oxide and mixtures thereof. The aggregated abrasive particles so produced are incorporated in resin bonded grinding wheels which are used for the grinding of steels and hard alloys.

According to this invention, there is provided a method of making aggregated diamond abrasive particles comprising forming a mixture of diamond abrasive particles selected from Types A, B and C, as hereinafter defined, and mixtures thereof and a powdered metal, heating the mixture to sinter the metal followed by cooling to produce a mass and crushing the mass to obtain the aggregated particles. The diamond abrasive particles which are used in the present invention are of a particular type and form an essential feature of the invention. The three types of diamond abrasive particles which are used in the method of the invention are characterised below.

Particles of Type A have the following characteristics:

1. A Friatest Index of about 70 to 90, preferably 77 to 87, for those particles of size 74 to 88 microns.

2. Medium metal content, i.e. a metal content of about 10 to 15 percent by weight.

3. Predominantly translucent colour with the occasional transparent white, grey and yellow particle

4. Predominantly blocky shape tending towards elongate particles.

5. Predominantly rough and undulating surface.

These particles are typically diamonds produced by the process described in U.S. Pat. No. 4,036,937, i.e. the particles remaining after the particles of length to width ratio of at least 2:1 and of size in the range 50 to 170 U.S. mesh have been removed from a batch of particles produced by this method.

Particles of type B have the following characteristics:

1. Weak, having a Friatest Index of about 65 to 88, preferably 80 to 88, for those particles of size 74 to 88 microns.

2. A high metal content, i.e. a metal content of greater than 2 percent by weight.

3. Predominantly dark in colour.

4. A blocky and irregular shape.

5. Rough with surface irregularities, re-entrant angles and etch pits.

These particles are typically of the resinoid diamond type.

Particles of type C have the following characteristics:

1. Medium strength, having a Friatest Index in the range about 100 to 121, preferably 111 to 121, for those particles of size 74 to 88 microns.

2. Low metal content, i.e. a metal content of less than 0.8 percent by weight.

3. Predominantly yellow in colour.

4. An angular to blocky shape with sharp edges.

5. A predominantly smooth surface.

These particles are typically diamonds of the metal bond type which have been rejected on the ground of quality or crushed such reject material. The Friatest Index is a measure of the strength or friability of abrasive particles and is obtained by the commercially available Friatest method. In this test, a sample of the abrasive particles of particular particle size is placed in a capsule with a hard steel ball and shaken for a predetermined period of time. The abrasive particles are then removed from the capsule and screened through the next smaller screen size in relation to the smaller of the two screens used for determining the screen size of the original particles. The amount retained on the screen divided by the weight of the original sample gives you a value R. The friability or Friatest Index (F.T.I.) of the particle may then be calculated from the following formula: ##EQU1## where t is the time of shaking in the capsule. The higher the F.T.I. the stronger or less friable the particle.

The diamond abrasive particles which are used as starting material in the manufacture of the aggregated particles may be of the Type A, of the Type B, of the Type C or mixtures thereof. For best results, as discussed more fully hereinbelow, it is preferred that a mixture of particles is used with Type A constituting at least 40 percent of the mixture, preferably 40 to 80 percent of the mixture, Types B and C constituting the balance of the mixture. Types B and C are typically and preferably provided in substantially equal proportions. A typical mixture is one containing 50 percent Type A and 25 percent of each of Types B and C. All percentages in these mixtures are percentages by weight.

A particular size of diamond abrasive particle may be used or a blend of sizes may be used. It is preferable that the particles used are all fine and in particular have a size smaller than about 125 microns.

As is illustrated more fully hereinafter, surprisingly good grinding results are obtained if the aggregated particles are within particular particle size ranges. It is preferred that the aggregated particles have a size in the range 149 to 250 microns, more preferably in the range 177 to 250 microns.

The binder metal may be any of a variety of metals or alloys. Examples of suitable metals are silver, copper, tin, nickel, cobalt and iron and alloys containing one or more such metals. The metal or alloy will preferably have a melting point below 1200° C. and preferably below 1000° C. Particularly suitable alloys are copper-silver and copper-tin alloys, and more particularly such alloys which are eutectic alloys. A 71:29:: copper:silver alloy is a eutectic alloy and a 80:20:: copper:tin alloy is a eutectic alloy. Where an alloy is employed, the alloy, in powdered form, may be used for the starting mixture. It is, however, preferred that the individual components for the alloy, in powdered form, are used for the mixture. The alloy in this case is produced in situ during sintering.

The size of the metal powder which is used is not critical. The powder will generally be fine, typically having a particle size of less than 100 microns.

The mixture of diamond abrasive particles and metal powder will usually contain a diamond-wetting agent such as titanium, zirconium, vanadium, chromium and silicon. The diamond-wetting agent is typically provided in an amount of 5 to 15 percent by weight of the metal powders.

The amount of metal powder used in the mixture will generally be such as to produce aggregated particles which contain 40 to 60, typically 55, percent by weight of metal.

The powdered mixture of diamond abrasive particles and metal is sintered and then cooled to produce a mass which is coherent and bonded. The sintering typically takes place at a temperature in the range 700° to 1200° C., preferably 900° to 950° C. The period of sintering will vary according to the nature of the metal component in the mixture, but is typically 10 to 20 minutes.

In order to minimise graphitisation of the diamond, the sintering and cooling is preferably carried out in a substantially non-oxidising atmosphere. The non-oxidising atmosphere may be a non-oxidising gas such as hydrogen, nitrogen or an inert gas such as argon or neon or the non-oxidising atmosphere may be provided by a vacuum which is typically 10⁻⁴ Torr or better.

The sintering and cooling may take place with or without compaction of the mixture. Where there is no compaction, that is the powders are in a substantially loose bulk state, a more porous mass is obtained. Compaction, when used, should not be too severe so as to avoid producing too compact a mass.

The crushing of the particles is preferably by shear, rather than impact crushing. Any shear crushing method known in the art may be used. For example, the sintered mass may be crushed to the desired particle size in a jaw or gyro crusher.

The aggregated diamond abrasive particles produced by the above described method consist of a number of diamond abrasive particles held together by the metal. They may be used in abrasive tools, particularly resin-bonded abrasive tools such as grinding wheels. Prior to incorporation in the wheel the aggregated particles may be provided with a thin metal coat.

The abrasive tools incorporating the aggregated diamond abrasive particles may be of conventional construction. In the case of resin-bonded grinding wheels the abrasive particles will generally be present in the grinding portion of the wheel in an amount of 15 to 30, typically 20, percent by volume of that portion. The resin may be any suitable thermosetting resin known in the art of resin-bonded grinding wheels such as a phenolformaldehyde, polyamide, polyimide, urea-formaldehyde, polyester or melamine-formaldehyde resin. The construction of the wheel may be of the flaring cup configuration or of the peripheral configuration. As is known, grinding wheels consist of a central hub portion to which is bonded a peripheral grinding portion.

The aggregated diamond abrasive particles when incorporated in resin-bonded abrasive tools have been found to be particularly effective in abrading, particularly under dry conditions, hard materials such as cemented carbides, nitrides and abrasive compacts. The invention thus provides, according to another aspect, a method of abrading a workpiece of the type described above including the steps of providing an abrasive tool, the abrading surface of which comprises aggregated diamond abrasive particles, as produced above, incorporated in a resin matrix, abrading the workpiece by moving the abrading surface of the tool relative to a surface of the workpiece and contacting the relatively moving surfaces. Generally, the abrasive tool will be a grinding wheel which will be rotated and the rotating grinding wheel contacted with the workpiece to effect grinding.

In particular, effective grinding of cemented carbides, particularly tungsten carbide, is achieved in the manner described above. Cemented carbides, as is known in the art, consist of a mass of carbide particles such as tungsten carbide, tantalum carbide or titanium carbide particles bonded into a hard, coherent mass with a metal bonding matrix. The metal is generally cobalt, nickel or iron or an alloy thereof and is generally present in the cemented carbide in an amount of 6 to 35 percent by weight. Abrasive compacts are well known in the art and consist essentially of a mass of abrasive particles, generally present in an amount of at least 70%, preferably 80 to 90%, by volume of the compact, bonded into a hard conglomerate. Compacts are polycrystalline masses and can replace single large crystals. The abrasive particles of compacts are invariably super-hard abrasives such as diamond and cubic boron nitride.

The invention is further illustrated by the following nonlimiting examples.

EXAMPLE 1

Particles of type A were mixed with a commercially available copper/silver eutectic alloy and titanium in powder form. The mixture was made up of 100 parts by weight diamond, 90 parts by weight of the silver/copper alloy and 7 parts by weight titanium. The mixture was heated to a temperature of about 900° C. and then cooled. The heating and cooling took place in a vacuum of 10⁻⁴ Torr and produced a fused mass. The mass was crushed in a jaw crusher to produce aggregated diamond abrasive particles of size in the range 177 to 250 microns.

The aggregated diamond abrasive particles were incorporated into a phenolfoamaldehyde resin bonded grinding wheel containing 20 percent by volume of the abrasive particles. The grinding wheel was of the flaring cup, i.e. 11V9 configuration and had a 100 mm diameter and a 3 mm thickness. The grinding wheel was made in the conventional manner by molding to a stop. The grinding wheel was used to dry grind a K21 tungsten carbide workpiece at a peripheral speed of 17 meters/sec., an infeed of 0,03 mm and a table traverse speed of 3 meters/min. The G-ratio obtained was 158.

By way of comparison, aggregated diamond abrasive particles were produced using the same method except that commercially available RD diamond particles were used. The aggregated particles were incorporated in a similar 11V9 grinding wheel and the wheel was used to dry grind a K21 carbide workpiece under similar conditions. The G-ratio obtained with this wheel was only 117.

The G-ratio is the ratio of the amount of workpiece removed to the amount of wheel consumed during grinding. The higher the G-ratio of the better the wheel.

EXAMPLE 2

Further batches of aggregated diamond particles were produced in a similar manner to Example 1, save that in each case separate copper and silver powders were used in the starting mixture. The aggregated particles produced were incorporated into grinding wheels. The grinding wheels were used to grind tungsten carbide and compared with similar wheels incorporating a commercially available copper-clad diamond grit (which is sold specifically for the dry grinding of carbides). The results of these tests are set out in Tables I and II. The following should be noted in relation to these tests:

The concentration of particles is represented as 75 concentration. This is equivalent to a concentration of 20 percent by volume.

It is apparent from the G-ratios obtained, that wheels incorporating aggregated diamond abrasive particles produced by the method of the invention are superior to wheels incorporating commercially available copper clad grit.

EXAMPLE 3

A number of other tests were carried out to compare aggregated particles using the prescribed particles with aggregated particles using other diamond particles; to evaluate the effect of the amount of metal present in the aggregated particles and to evaluate various sizes of aggregated particles. The procedures followed were similar to those set out in Examples 1 and 2. The results obtained are set out below.

(i) Variations in Diamond Particle

Diamond particles of various types were used as starting material and the G-ratios obtained for the various tests are set out below:

    ______________________________________                                                         Dry         Wet                                                Diamond Type    Grinding - K21                                                                             Grinding - K3H                                     ______________________________________                                         MD              18          40                                                 DXDA-II (an MD particle)                                                                       30          53                                                 Type C          42          73                                                 Type A          62          104                                                Type B          35          65                                                 ______________________________________                                    

(ii) Variations in Metal Content

The metal content of the aggregated particles was varied. In each case a mixture consisting of 50 percent by weight Type A and 25 percent by weight of each of Types B and C was used in the starting mixture. The results obtained are set out below:

    ______________________________________                                         Percentage Metal in Aggregate                                                                   G-Ratio                                                       ______________________________________                                         40               44                                                            50               50                                                            60               48                                                            ______________________________________                                    

(iii) Variations in Size of Aggregated Particles

The aggregated particles were crushed to various sizes and then the G-ratios obtained using these various sizes compared. Again, a mixture as specified in (ii) above was used in the starting mixture. The G-ratios obtained are set out below:

    ______________________________________                                         Size        G-ratio as Percentage of Size 1                                    ______________________________________                                         1. 177-250 microns                                                                         100                                                                2. 149-177 microns                                                                         94                                                                 3. 125-149 microns                                                                         77                                                                 4. 105-125 microns                                                                         58                                                                 5.  88--105 microns                                                                        60                                                                 ______________________________________                                    

EXAMPLE 4

A powdered mixture as in Example 1, save that a mixture of silver and copper powders was used, was placed in a mold and compacted under a load of 24 gm/cm². The resulting compacted mass was sintered and cooled as in Example 1. The coherent mass produced was found to have a 58.1 percent porosity. Using the same starting mixture and same procedure, save that there was no compaction of the mixture, a second coherent mass was produced. This mass was found to have a 61,5 percent porosity.

The two masses were crushed as in Example 1 to produce two batches of aggregated diamond abrasive particles. The two batches were incorporated in different phenolformaldehyde resin bonded grinding wheels and the wheels tested in the dry grinding of tungsten carbide. The two wheels showed no material difference in grinding performance.

                                      TABLE I                                      __________________________________________________________________________             TYPE              TACHELLA                                                     TABLE TRAVERSE    2,0 m/min.                                                   CROSSFEED                                                              MACHINE INFEED            0,03 mm.                                                     SPINDLE SPEED     3,200                                                        TOTAL INFEED      1,8 mm.                                              COOLANT TYPE AND DILUTION DRY                                                          FLOW RATE                                                              WORKPIECE                                                                              MATERIAL          K21 tungsten carbide                                         SIZE              1/2" × 1/4"                                                                         24 PIECES                                         SIZE AND TYPE     100 × 3 mm                                                                          11V9                                      WHEEL   PERIPHERAL SPEED  17 m/sec.                                                    BOND              1   2     3   4                                              NUMBER                                                                         TYPE              B          Copper-clad                                                                    grit                                      GRIT    SIZE(MICRONS)     177-250    177-250                                           CONCENTRATION     75         75                                                POWER             1120                                                                               1080  1080                                                                               1240                                                             1080                                                                               1040  1120                                                                               1040                                                             1040                                                                               1040  1120                                                                               1040                                           Average Power per Wheel                                                                          1060                                                                               1040  1120                                                                               1040                                   RESULTS AVERAGE POWER PER GRIT                                                                           1050       1080                                              G RATIO           87  127    56 78                                                               83  93     58 53                                                               90  67     49 62                                             Average G Ratio for                                                            Wheel             87  80     54 57                                             AVERAGE G RATIO FOR GRIT                                                                           84          56                                     __________________________________________________________________________

                                      TABLE II                                     __________________________________________________________________________             TYPE              KELLENBERGER                                                 TABLE TRAVERSE    2,0 m/min                                                    CROSSFEED                                                              MACHINE INFEED            0,02 mm.                                                     SPINDLE SPEED     3200                                                         TOTAL INFEED      1,6 mm.                                              COOLANT TYPE AND DILUTION                                                              FLOW RATE         DRY                                                  WORKPIECE                                                                              MATERIAL          K21                                                          SIZE              1/2" × 1/4"                                                                         24 PIECES                                         SIZE AND TYPE     100 × 3 mm.                                                                         11V9                                      WHEEL   PERIPHERAL SPEED  17 m/sec.                                                    BOND                                                                           NUMBER            5     6    7                                                 TYPE              A     B    Copper-clad                                                                    grit                                      GRIT    SIZE(MICRONS)     177/250                                                                              177/250                                                                             177/250                                           CONCENTRATION     75    75   75                                                POWER             1160  1120 1280                                                                1120  1000 1080                                                                1160  800  1160                                              Average Power per Wheel                                                                          1140                                                 RESULTS AVERAGE POWER PER GRIT                                                                           1140  947  1093                                              G RATIO           63    37   28                                                                  67    47   27                                                                  57    43   34                                                                        58                                                     Average G Ratio for                                                            Wheel             62    49   33                                                AVERAGE G RATIO FOR GRIT                                                                         62    49   33                                        __________________________________________________________________________ 

I claim:
 1. A method of making aggregated diamond abrasive particles comprising forming a mixture of diamond abrasive particles and a powdered metal, heating the mixture to sinter the metal followed by cooling to produce a mass and crushing the mass to obtain the aggregated particles, characterised in that the particles comprise particles of size 74 to 88 microns and the particles are selected from Types A, B and C, as hereinafter defined, and mixtures thereof:Particles of Type A have the following characteristics:1. A Friatest Index of about 70 to 90, for those particles of size 74 to 88 microns;
 2. A metal content of about 1.0 to 1.5 percent by weight;
 3. Predominantly translucent colour with the occasional transparent white, grey and yellow particle;
 4. Predominantly blocky shape tending towards elongate particles;
 5. Predominantly rough and undulating surface; Particles of Type B have the following characteristics:1. Weak, having a Friatest Index of about 65 to 88, for those particles of size 74 to 88 microns;
 2. A metal content of greater than 2 percent by weight;
 3. Predominantly dark in colour;
 4. A blocky and irregular shape;5. Rough with surface irregularities, re-entrant angles and etch pits; Particles of Type C have the following characteristics:1. Medium strength, having a Friatest Index in the range about 100 to 121 for those particles of size 74 to 88 microns;
 2. A metal content of less than 0.8 percent by weight;
 3. Predominantly yellow in colour;
 4. An angular to blocky shape with sharp edges;
 5. A predominantly smooth surface.
 2. A method according to claim 1 characterised in that the particles are a mixture of type A particles and particles of at least one type selected from the group consisting of type B and C, type A particles constituting at least 40 percent by weight of the mixture.
 3. A method according to claim 2 wherein the particles of type A constitute 40-80 percent by weight of the mixture.
 4. A method according to claim 2 wherein the mixture contains particles of type A, type B and type C, the particles of types B and C being present in substantially equal proportions.
 5. A method according to claim 1 wherein the particles are a mixture consisting of 50 percent type A particles, and 25 percent of each of types B and C, all percentages being by weight.
 6. A method according to claim 1 wherein the diamond abrasive particles which are used in the starting mixture have a size smaller than about 125 microns.
 7. A method according to claim 1 wherein the aggregated particles have a size in the range 149-250 microns.
 8. A method according to claim 4 wherein the aggregated particles have a size in the range 149-250 microns.
 9. A method according to claim 1 wherein the aggregated particles have a size in the range 177-250 microns.
 10. A method according to claim 4 wherein the aggregated particles have a size in the range 177-250 microns.
 11. A method according to claim 1 wherein the metal is selected from silver, copper, tin, nickel, cobalt and iron and alloys containing one or more of these metals.
 12. A method according to claim 11 wherein the metal or alloy has a melting point below 1200° C.
 13. A method according to claim 11 wherein the metal or alloy has a melting point below 1000° C.
 14. A method according to claim 1 wherein the metal powder consists of a powdered copper/silver alloy.
 15. A method according to claim 8 wherein the metal powder consists of a powdered copper/silver alloy.
 16. A method according to claim 1 wherein the powdered metal consists of a mixture of copper and silver powders.
 17. A method according to claim 8 wherein the powdered metal consists of a mixture of copper and silver powders.
 18. A method according to claim 1 wherein the mixture of diamond abrasive particles and metal powder also contains a diamond wetting agent.
 19. A method according to claim 8 wherein the mixture of diamond abrasive particles and metal powder also contains a diamond wetting agent.
 20. A method according to claim 18 wherein the diamond wetting agent is selected from titanium, zirconium, vanadium, chromium and silicon.
 21. A method according to claim 18 wherein the diamond wetting agent is present in an amount of 5-15 percent by weight of the metal powders.
 22. A method according to claim 1 wherein the amount of metal powder in the starting mixture is such as to produce aggregated particles containing 40-60 percent by weight of metal.
 23. A method according to claim 1 wherein the amount of metal powder used in the starting mixture is such as to produce aggregated particles containing 55 percent by weight of metal.
 24. A method according to claim 1 wherein sintering takes place at a temperature in the range 700° to 1200° C.
 25. A method according to claim 24 wherein the sintering takes place at a temperature in the range 900° to 950° C.
 26. A method according to claim 1 wherein sintering takes place for a period of 10 to 20 minutes.
 27. A method according to claim 1 wherein the sintering and cooling takes place in a substantially non-oxidising atmosphere.
 28. A method according to claim 27 wherein the non-oxidising atmosphere is selected from a non-oxidising gas and a vacuum of 10⁻⁴ Torr or better.
 29. A method according to claim 1 wherein the crushing is effected by shear crushing.
 30. Aggregated diamond abrasive particles when produced by a method according to (any one of the preceding claims) claim
 1. 